Theoretical and experimental bases are given for measuring the complex forward-scattering amplitude of single particles through self-reference interferometry. Our analyses reveal the nondimensional ...parameters that primarily control the accuracy and resolution of the complex amplitude data. We propose a measurement protocol, Complex Amplitude Sensing version 1 (CAS-v1), for effectively utilizing self-reference interferometry as a universal tool for inline measurements of the complex forward-scattering amplitude of single sub- and super-micron particles suspended in a fluid flow. The CAS-v1 protocol will facilitate applications of self-reference interferometry to real-time particle measurements in the industrial, biomedical, and environmental sciences.
•A non-invasive optical particle-characterization method for submicron particles.•Accurate measurement of real and imaginary parts of complex refractive index.•Accurate measurement of ...volume-equivalent particle-size distribution.•Applicable to both spherical and non-spherical particles•A novel Bayesian computational approach for solving inverse scattering problem.
Nondestructive inline measurements of submicron-sized particles and gas bubbles are required in many fields of study, including environmental, industrial, and biomedical research. The single particle extinction and scattering (SPES) method is a promising technique for this purpose because of its sensitivity to both the complex refractive index and the size of individual particles. Herein an original modification of the SPES instrument that suppresses measurement bias and background noise is presented. In addition, relevant theory and a practical algorithm for data analysis are presented. The inverse scattering problem of estimating the physical characteristics of a particle from SPES data using Bayesian theory for model selection is formulated, and the performance of the algorithm using simulated data for spherical and non-spherical particles is evaluated. The results suggest that a cluster of SPES data points for an ensemble of particles contains sufficient information to retrieve the complex refractive index and volume equivalent size-distribution of the particles, regardless of particle shape. The Bayesian inversion method also allows incorporation of informative data from other analytical methods (e.g., electron microscope, upstream size-sorting device) through the model and parameter priors. As practical examples, the SPES method was used to constrain the complex refractive index and size distribution of water-insoluble aerosols contained in rainwater. The results suggest that the SPES method itself or its use in combination with other conventional analytical methods can quantify the complex refractive index and size distribution of almost any type of submicron particle (including gas bubbles) in environmental, industrial, and biomedical samples.
Light-absorbing aerosols affect atmospheric radiation, dynamics, and precipitations through shortwave absorption in the atmosphere and snowpack. Black carbon (BC) is considered the most significant ...contributor to global shortwave absorption among all the known light-absorbing aerosol components. In analyses and predictions of BC’s lifecycle and climate effects, multiscale field observations are needed to test the fundamental assumptions in the climate model. In situ measurements, the focus of this review, fill the gap of observational information accessible from remote sensing and laboratory analyses. This article reviews historical backgrounds, recent advances in in situ measurements of BC, and the resulting observational findings used to update the assumptions in climate models and remote sensing. Finally, we raise open problems that demand a rethinking and future investigation.
Illustrating the physical principle of detecting the light-absorbing black carbon and iron oxides aerosol particles using the single-particle laser-induced incandescence
An accurate and efficient simulation of light scattering by an atmospheric black carbon (BC)-containing aerosol—a fractal-like cluster of hundreds of carbon monomers that is internally mixed with ...other aerosol compounds such as sulfates, organics, and water—remains challenging owing to the enormous diversities of such aerosols׳ size, shape, and mixing state. Although the discrete dipole approximation (DDA) is theoretically an exact numerical method that is applicable to arbitrary non-spherical inhomogeneous targets, in practice, it suffers from severe granularity-induced error and degradation of computational efficiency for such extremely complex targets. To solve this drawback, we propose herein a hybrid DDA method designed for arbitrary BC-containing aerosols: the monomer-dipole assumption is applied to a cluster of carbon monomers, whereas the efficient cubic-lattice discretization is applied to the remaining particle volume consisting of other materials. The hybrid DDA is free from the error induced by the surface granularity of carbon monomers that occurs in conventional cubic-lattice DDA. In the hybrid DDA, we successfully mitigate the artifact of neglecting the higher-order multipoles in the monomer-dipole assumption by incorporating the magnetic dipole in addition to the electric dipole into our DDA formulations. Our numerical experiments show that the hybrid DDA method is an efficient light-scattering solver for BC-containing aerosols in arbitrary mixing states. The hybrid DDA could be also useful for a cluster of metallic nanospheres associated with other dielectric materials.
•Light scattering code is developed for atmospheric black carbon aerosols.•A hybrid discretization scheme in discrete dipole approximation (DDA) is proposed.•The addition of the magnetic dipole improves the accuracy of the hybrid DDA.•Improved computational efficiencies are achieved in the hybrid DDA.
The representation of aerosol activation into cloud droplets in climate models is important for accurate understanding of aerosol radiative impacts on the Arctic climate, but it remains highly ...uncertain. Here we show that the uncertainty range of subgrid vertical velocity (SVV) and maximum supersaturation (SSmax) in aerosol activation produces fourfold to fivefold differences in the direct radiative effect of black carbon (BC) in the Arctic (0.091–0.40 W m−2) because SVV and SSmax determine the activated fraction and wet removal efficiency of aerosols. Aerosols are particularly sensitive to SVV in remote regions but not near their sources because many aerosols near sources are not yet influenced by wet removal processes. Our results demonstrate that SVV treatment is a major source of uncertainty in Arctic aerosol simulations and may be key for reducing the large discrepancies among global models in estimates of BC and its radiative effects in the Arctic.
Plain Language Summary
Black carbon aerosol, emitted mainly in midlatitude regions by combustion of fossil fuel and biomass, is transported to the Arctic and deposited on snow and ice surfaces, where it contributes to Arctic heating. However, estimates of its importance in Arctic warming have large uncertainties. Because global climate models usually use a coarse horizontal grid spacing, they rely on many assumptions to represent the small‐scale atmospheric processes within clouds. This study uses a global climate model to investigate the importance of one of these assumptions, the treatment of “subgrid vertical velocity,” to aerosol simulations. We show that varying the subgrid vertical velocity within its uncertainty range changes the calculated heating effect of black carbon in the Arctic by as much as five times. Our results underscore the importance of treating subgrid vertical velocity treatment accurately in estimating how much black carbon from midlatitudes is warming the Arctic.
Key Points
The importance of subgrid vertical velocity in activation to aerosol burden and radiative effects was investigated by using a global model
The current uncertainty in subgrid vertical velocity produces fivefold differences in the radiative effect of black carbon in the Arctic
The subgrid treatment of updraft is important in estimating the long‐range transport of black carbon and its impacts on Arctic climate
We used a single-particle soot photometer (SP2) to measure the mass of individual black carbon (BC) particles down to ∼ 0.5 fg by means of laser-induced incandescence with an intra-cavity, ...continuous-wave laser. The incandescence of nine different types of BC samples was investigated to provide a physical basis for choosing appropriate BC materials for SP2 calibration. We estimated the vaporization temperatures of these BC samples from the spectral dependence of incandescence at the limit of the small size parameter x, for which spectral dependence of emissivity is known a priori. The vaporization temperatures differed by less than 2.2% among the samples. For the x < 1 regime of particle size, the peak amplitude of the incandescence signal measured by the SP2 was linearly proportional to the particle mass. The slopes of such linear proportionality were positively correlated with | (m
2
-1)/(m
2
+2)|, where the m is the complex refractive index of the BC particle. For particles in which x > 1, the rate of increase in the peak amplitude of the incandescence signal with increasing particle mass was negatively correlated with the compactness of particle shape, consistent with the theoretical prediction of emissivity, which accounts for particle shape. The incandescence-BC mass relationships were similar between fullerene soot and ambient soot sampled in Tokyo, thus suggesting that fullerene soot is a suitable calibration standard for SP2 measurements of ambient soot.
Light‐absorbing atmospheric aerosols such as carbonaceous particles influence the climate through absorbing sunlight. The mixing states of these aerosol particles affect their optical properties. ...This study examines the changes in the mixing states and abundance of strongly light absorbing carbonaceous particles by using transmission electron microscopy (TEM) and single‐particle soot photometer (SP2), as well as of iron oxide particles, in Tokyo, Japan. TEM and SP2 use fundamentally different detection techniques for the same light‐absorbing particles. TEM allows characterization of the morphological, chemical, and structural features of individual particles, whereas SP2 optically measures the number, size, and mixing states of black carbon (BC). A comparison of the results obtained using these two techniques indicates that the peaks of high soot (nanosphere soot (ns‐soot)) concentration periods agree with those of the BC concentrations determined by SP2 and that the high Fe‐bearing particle fraction periods measured by TEM agree with that of high number concentrations of iron oxide particles measured using SP2 during the first half of the observation campaign. The results also show that the changes in the ns‐soot/BC mixing states primarily correlate with the air mass sources, wind speed, precipitation, and photochemical processes. Nano‐sized, aggregated, iron oxide particles mixed with other particles were commonly observed by using TEM during the high iron oxide particle periods. We conclude that although further quantitative comparison between TEM and SP2 data will be needed, the morphologically and optically defined ns‐soot and BC, respectively, are essentially the same substance and that their mixing states are generally consistent across the techniques.
Key Points
Occurrences of ns‐soot/BC from TEM and SP2 data are generally consistent across the techniques
Abundance and mixing state of ns‐soot/BC fractions changed after rain and during weak wind period
Aggregated‐Fe particles were found during high iron oxide particles concentration periods
Black carbon is the largest contributor to global aerosol's shortwave absorption in the current atmosphere and is an important positive climate forcer. The complex refractive index, m = m
r
+ im
i
, ...the primary determinant of the absorbed and scattered energies of incident radiation per unit volume of particulate material, has not been accurately known for atmospheric black carbon material. An accurate value at visible wavelengths has been difficult to obtain due to the black carbon's wavelength-scale irregularity and variability of aggregate shape, distribution in particle size, and mixing with other aerosol compounds. Here, we present a method to constrain a plausible (m
r
, m
i
) domain for black carbon from the observed distribution of the complex forward-scattering amplitude S(0°). This approach suppresses the biases due to the above-mentioned complexities. The S(0°) distribution of black carbon is acquired by performing single particle S(0°) measurements in a water medium after collecting atmospheric aerosols into water. We demonstrate the method operating at λ = 0.633 μm for constraining the refractive index of black carbon aerosols in the north-western Pacific boundary layer. From the plausible (m
r
, m
i
) domain consistent with the observed S(0°) distributions and the reported range of mass absorption cross-section, we conservatively select 1.95 + 0.96i as a recommendable value of the refractive index for uncoated black carbon at visible wavelengths. The recommendable value is 0.17 larger in m
i
than the widely used value 1.95 + 0.79i in current aerosol-climate models, implying a ∼16% underestimate of shortwave absorption by black carbon aerosols in current climate simulations.
Combustion-induced carbonaceous aerosols, particularly black carbon (BC) and brown carbon (BrC), have been largely considered as the only significant anthropogenic contributors to shortwave ...atmospheric heating. Natural iron oxide (FeO
) has been recognized as an important contributor, but the potential contribution of anthropogenic FeO
is unknown. In this study, we quantify the abundance of FeO
over East Asia through aircraft measurements using a modified single-particle soot photometer. The majority of airborne FeO
particles in the continental outflows are of anthropogenic origin in the form of aggregated magnetite nanoparticles. The shortwave absorbing powers (P
) attributable to FeO
and to BC are calculated on the basis of their size-resolved mass concentrations and the mean P
(FeO
)/P
(BC) ratio in the continental outflows is estimated to be at least 4-7%. We demonstrate that in addition to carbonaceous aerosols the aggregate of magnetite nanoparticles is a significant anthropogenic contributor to shortwave atmospheric heating.
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